A vinylidene fluoride polymer is a crystalline polymer and is used for providing various shaped products as a polymer having good mechanical strength. In this instance, for the purpose of providing such a shaped product with a good size stability suitable for the intended use, it has been generally practiced to subject the shaped product before the use thereof to a sufficient heat treatment (hereinafter sometimes called “aging”) so as to remove a strain during the shaping and cause additional crystallization. After the aging, however, the shaped product is liable to color into yellow or brown sometimes, thus lowering the commercial value thereof. Accordingly, a hardly colorable vinylidene fluoride polymer resin has been desired, and some commercial products of improved colorability have been put on the market, but further improvements have been desired.
As processes for producing vinylidene fluoride polymers, there have been known solution polymerization, emulsion polymerization and suspension polymerization. Solution polymerization is generally performed at a polymerization temperature of 20-80° C. and at a relatively low pressure of, e.g., at most 1.5 MPa, for providing a polymer used for paint having a relatively low molecular weight of, e.g., a number-average molecular weight of at most 105 corresponding to at most ca. 0.5 dl/g in terms of an inherent viscosity described hereinafter, and other polymerization processes are used for producing vinylidene fluoride polymers of higher molecular weights used for shaped products, etc. Further, because of a strong hydrogen-abstracting power of growing vinylidene fluoride radicals, the solvents usable therefor are restricted, and ketone solvents or acetate ester solvents (Patent document 1 listed below), chlorofluoroethane solvents (Patent document 2 below), etc., have been used.
In emulsion polymerization, a chemically stable fluorine-containing emulsifier or buffer agent is used for the polymerization to obtain a polymerizate comprising a latex having small particle sizes of 0.2-0.5 μm, which is then treated with a flocculant, etc., for precipitation and particle formation to obtain polymer particles. Emulsion polymerization has an advantage of a rapid polymerization speed, but is accompanied with difficulties in convenience and production cost due to the use of an expensive emulsifier, and also difficulties, such as powder recovery through agglomeration and precipitation, and removal of ionic impurities originated from the flocculant.
Suspension polymerization is a process which has been practiced since development of initiators exhibiting polymerization initiation ability at low temperatures so as to make possible polymerization at a temperature below 30.1° C. (i.e., critical temperature of vinylidene fluoride monomer). Vinylidene fluoride monomer alone or together with a copolymerizable monomer is dispersed within water, and the resultant monomer droplets are subjected to polymerization in the presence of a polymerization initiator. It is also possible to start the suspension polymerization at a temperature below 30.1° C. and then continue the polymerization by raising the temperature after formation of the particles. The suspension polymerization allows easy post-processing of the resultant polymer and is most suited for providing a vinylidene fluoride polymer excellent in processability, mechanical properties and thermal stability.
It is well known that the selection and used amount of a suspension agent, a chain transfer agent and a polymerization initiator have influences on the coloring characteristic of a vinylidene fluoride polymer obtained in the polymerization and a shaped product thereof. For example, a suspension polymerization process for a vinylidene fluoride polymer less liable to be colored by using a specific chain transfer agent (chain-adjusting agent), such as bis(ethyl) carbonate, has been proposed (Patent document 3 below). However, even in, the case of using such a chain transfer agent, the resultant vinylidene fluoride polymer has left a room for improvement regarding the durability, and also difficulties, such as the necessity of a high polymerization pressure and scaling on the gaseous phase wall in the polymerization vessel due to gaseous-phase polymerization.
Further, it has been also proposed to produce a vinylidene fluoride polymer improved in high-temperature coloring resistance and low elutability by using a special halide solvent which shows good dissolving power to both vinylidene fluoride monomer and polymerization initiator and also shows a low chain transfer effect, to reduce the used amount of the polymerization initiator (Patent document 4 below). The process surely provides a vinylidene fluoride polymer improved in high-temperature coloring resistance and low elutability, but is accompanied with a problem requiring troublesome labor and cost for recovering a relatively expensive solvent. Also, a further overall improvement in high-temperature resistance and low elutability is desired.    Patent document 1: JP-A 59-135257,    Patent document 2: JP-A 6-322028,    Patent document 3: JP-B 3-48924,    Patent document 4: JP-A 2002-220403.